M-Audio make some nice products in this area, e.g.
http://www.m-audio.com/products/en_u...eUSB-main.html
Digigram make a large range of professional-quality soundcards, the VXPocket PCMCIA cards are populare for mobile use.
http://www.digigram.com/products/index.htm
RME Audio also make professional-quality souncards,
http://www.rme-audio.com/english/soundcds.htm